Radio-frequency (RF) amplifiers with the common industrial frequencies of 13.56 MHz and 27.12 MHz and output powers of 1 kW to 50 kW are conventionally used in the field of laser excitation or plasma processes.
Load impedances of laser excitation or plasma processes can be non-linear and dynamic, exhibiting unpredictable changes. These dynamic impedance changes generate reflections that produce losses in the RF amplifier. Large reactive energies that are stored in the reactive elements of the RF amplifiers, in the feed lines, and in reactive elements of matching networks can thereby be discharged. Such discharge may generate high voltages or currents, may excite oscillations in the RF amplifier, or may destroy components of the RF amplifier. Such load impedance changes may occur, for example, during striking of the laser excitation or plasma processes, or during arcing in the plasma process.
Radio-frequency operated laser excitations, and, to an increasing extent, also radio-frequency excited plasma processes, can be operated in a pulsed manner, i.e., the radio frequency amplifiers are switched on and off with pulse frequencies of, e.g., 100 Hz to 300 kHz, or are switched between two power ranges. Temporary reflections may be produced during each switching process, and these temporary reflections may be converted into lost energy, that may accumulate as excess heat in the RF amplifiers.
The output stages of such RF amplifiers may be realized with transistors for small powers (for example, 1–6 kW). For larger power, tubes may be used as output stages of RF amplifiers. Tubes can be more robust to reflections and can discharge lost energy more effectively than transistors. Tubes, however, can be more expensive than transistors, can be subjected to wear during operation, and can be relatively large. Tube RF amplifiers can be bundled together with a drive circuit and cooling system in switching cabinets of a size of approximately 0.8 m×1 m×2 m.
RF amplifiers of a greater power may be formed with transistor output stages. The use of transistorized amplifiers has increased the use of switched amplifiers that operate in resonance mode. The transistors are thereby switched to produce a minimum or low amount of lost energy. In this manner, it is possible to construct amplifiers having very small dimensions and a comparatively large power. It is possible to construct 13.56 MHz 3 kW amplifiers of a size of approximately 0.3 m×0.2 m×0.2 m. Integration of these amplifiers in plasma systems or into laser excitation arrangements is facilitated due to their size.
Transistorized output stages can produce great power by interconnecting several synchronously operating RF amplifiers. The RF amplifiers are interconnected using so-called combiners. There are different types of construction of these combiners.
EP0962048B1 discloses, e.g., interconnection of several radio-frequency power amplifiers that is realized by so-called transmission-line combiners.
EP0731559 describes the interconnection of two RF amplifiers by way of a 0° combiner, in which the input signal of an amplifier is phase-shifted, and the output signal of the same amplifier is also shifted, so that the phases at the input of the combiner are again in phase. Cables of defined length are proposed as phase shifters. The cables are approximately 4 m long for 13.56 MHz using a conventional cable with a phase speed of 0.69*c0 (where c0=speed of light in vacuum).
The so-called 90° hybrid is a combiner that is frequently used in microwave technology or in radio transmission technology. The 90° hybrid is also called a 3 dB coupler. The 90° hybrid includes a quadruple gate or four-port device, i.e., it has four gates or ports.
When the 90° hybrid is used as combiner, two RF power amplifiers having identical inner resistances, identical output frequencies, and output signals that are phase-shifted by 90° are connected to one gate each. A load with a load resistance is connected to a third gate. A load compensating resistance is connected to the fourth gate.
The load resistance, load compensating resistance, and inner resistances of the amplifiers are the same. The exclusively passive components of the 90° hybrid (lines, capacitances, transmitters, or inductances) are designed in such a manner that the power of the two amplifiers is combined at the load, no power is dissipated at the load compensating resistance, and the two amplifiers are decoupled and cannot influence each other. The 90° hybrid itself is loss-free in the ideal case, i.e., the whole power of the two RF amplifiers is supplied to the load applied to the third gate.
DE 1143873 (GB0966629) discloses an application for combining the outputs of two load amplifiers for a short-wave transmitter. The coupler includes four lines of line sections of a length of λ/4, and a shorter construction consisting of λ/8 lines with additional capacitors, where λ is the wavelength of the transmitted medium frequency. For example, λ is 15.27 m for a medium frequency of 13.56 MHz with a common cable having a phase speed of 0.69*c0 (where c0=light speed in vacuum). In this example, λ/8 is almost 2 m, which is rather large for conventional amplifier sizes.